Subsurface prospecting



MalCh 28 1944. vn G. c. HARE SUBSURFAQE PROSPECTING Filed June 26, 1940 FIG. 2

DoNAL'IS G C. HARE INVENTORA HIS ATTORNEYS quently not reliable.

-proved to be sufficiently Patented Mar. 28, 1944 SUBSURFACE PROSPECTING Donald G. C. Hare, Houston, Tex., assignorto Texaco Development Corporation, New York, N. Y., a corporation of Delaware Application June 2651940, Serial No. 342,423

6' Claims.

This invention relates tothe prospecting of the subsurface of the earth and more particularly to a method and an apparatus to be used in determining the nature of the formations through which a bore hole or well has been drilled, The principal object of the invention is to provide such a method and apparatus which will indicate positively and accurately the presence of hydrocarbon oil sands or oil bearing for; mations'when the device is passed through the hole within these formations. It is also an object of the invention to provide a method which can be carried out with a minimum consumption of time and effort.

When drilling a bore hole for exploration purposes or for a well, it is of course desirable to know the depth at which are encountered those strata or formations which may contain oil in quantities of commercial interest. In the past. eiorts have been made to obtain this informa- .tion by various means such as the taking of core samples'or side wall samples. Electrical logging methods have also been used as Well as a method involving the measuring of the natural radioactive Ibackground of the formations. The taking oi core samples is obviously slow and expensive and the data obtained are fre- Electrical methods have been used in logging uncased holes but .these methods are not capable of yielding any pertinent data when used in a hole having the usual metal casing. The method involving the measurement of natural radioactivity has not as yet reliable to be commercially feasible.

In accordance with the present invention a device has been provided for making a continuous survey of a bore hole and since the opera-. tion of lthe device is not iniluenced by the-presence of iron or steel in the hole, it is well adapted to the surveying of cased holes as Well as those having no casing. Furthermore, the device could be used when lowered directly through a drill string'or pipe.

In my co-pending application, Serial No.

337,862, `led May 29, 1940, a method and appa-v ratus are disclosed by means of which fast neutrons are caused to be emitted and passed into the format-ion around the hole. Some of the neutrons after being scattered in the formation return to the surveying device to cause an indication ofthe variation in the types of formation through which the hole is drilled. In carrying out the present invention, a vsourceA of penetrative gamma rays which may be associated (Cl. Z-83.6)

with' a neutron'source is lowered into the hole and these rays, are scattered by the formations through which the hole hasbeen drilled, a certain number of these rays then returning to the instrument. 'I'he instrument is provided with a suitable detector such as a Geiger-Muller counter or ionization 'chamben The amount of radiation scattered back to the instrument will vary with the type of formation adjacent the instrument according to well known physical laws and thus by measurement of the intensity of this scattered radiation information may be obtained as to the nature of the formations.

For a better description of the invention, reference may be had to the accompanying `drawing in which Figure 1 is a vertical sectional elevation through a portion of a. bore hole showing an instrument embodying one form of the invention, and

Figure 2 is a similar elevation showing the instrument as embodied in a modied form.

Referring to the drawing, a bore hole I 0 is shown as having been drilled through a formation I2. The surveying instrument comprises a substantially closed casing Il of any suitable material adapted -to be lowered and raised through a hole I0 by means of a string of tubing I6 or other suitable means such as a wire line or cable. Disposed within the casing I4 is a suitable source I8 of gamma rays and neutrons.

The source I8 may be a tube or other suitable container in which is sealed a mixture oi' a small amount of a radium sulfate or chloride and a considerably larger quantity of finely powdered beryllium metal. A detector 20 adapted for the detection of gamma rays only is disposed within the casing I4 and is separated from the source I8 by means of a suitable shield 22 of lead. The detector 20 used for detecting gamma rays to the exclusion of neutrons may be such a device as a Geiger-Muller tube, operated-at a potential suiiiciently high so that the electrons emitted by the interaction of a gamma quantum with the tube Wall will cause the tube to discharge. 4Within the upper part of the casing I4 may be disposed a detector 24 adapted for the detection of neutronsonly and this del tector is also separated from the source I8 by means of a. shield 26. It is understood, of course, that the shields 22 and 26 tend to prevent direct radiation from the source I8 from reaching the detectors 20 and 24. 'I'he shield 26 for preventing neutrons from passing directly from the source to the detector may, if the detector is such as to respond to slow neutrons only. be a sheet of cadmium or other substance or a comv bination of\cadmium and boron having a high capture cross-sectionfor slow neutrons. Or, if desired to have the detector respond to both fast and slow neutrons, it may consist `of:--say, to cm. of a hydrogenic material such as parailin, for slowing down the neutrons, followed by a layer of cadmium or other substance as described above. Such a thickness of parailln will slow down practically all the neutrons to an energy suitable for their capture by a cadmium shield. The detectors It and 2l, which.

as before stated, may be Geiger-Muller counters or ionization chambers or other detectors such as are wen known to those versed in the art, are connected to suitable indicating or recording instruments, not shown. at the surfacel by means of wires 28 and Il respectively.

In operation the instrumentis lowered through a cased on uncased bore hole. preferably at a uniform speed, and gamma rays emitted from the source I8 penetrate the formation surrounding the hole. These rays will be scattered within the formation and some will return to the detector 20.' Since the number of gamma rays scattered per unit volume.will depend on certain physical properties such as density and atomic number of the substances making up the formation, by measuring the number'of such gamma rays scattered back to the detector 20 we can obtain valuable infomation concerning the nature of the formation adjacent to the infstrument. Thus. as the -device is moved along Y may be made of a bore hole and, as pointed out hereinbefore, since the gamma rays are little iniluenced bythe presence of iron or steel in the hole. the device can be used with success in the logging of holes containing casing orY drill pipe.

If desired, the source Il may be such as will emit'both gamma rays and fast neutrons, and

in this case the detector 24 will pick-up those neutrons which are scattered and returned to the instrument in a manner similar. to that in which the detector 20 picks up the returning gamma rays, but with this arrangement addi# tional information may be obtained such as the presence or absence of hydrogenic substances in -the structure of the formations through which the hole has been drilled. 'This will be particu-y larly true if the detector 2l is capable of differentiating between fast and slow neutrons, thus detecting those which`have been slowed down by collision with hydrogen nuclei. Such a de-` `tector may be a boron-triiluoride ionizationr chamber.

According to well known quantum mechanical considerations, the penetrating power or hardastenia source :fw-mun the ming u u a detector u adapted to detect scattered neutrons. The source 3| is separated from the detector 84 by means ofa suitable lead shield It. Below the source 3l and separated therefrom by means of another shield 38 is a detector Il. suitable for detecting gamma rays mainly, andthls detector is surroundedby a shield 42 which may consist of a layer of any substance such as lead which has the property of selectivelyv absorbing gamma radiation. By "selective absorbing is meant absorption which is a function of -the wave length or hardness of the radiation. A detector M of gamma rays and similar to the detector 40 is varranged in the lower portion of the casing 32 and the detector is not surrounded by any shield. Preferably. thetwo detectors or counters 40 and M are placed in near coincidence and thus receive essentially the same intensity of scattered gamma radiation. The shield 42 around the detector 4U will, however. decrease the intensity of the less penetrating gamma radiation and thus permit the detector 4l to record principally the radiation which has been least ail'ected by the scattering process. By coma different thickness of an absorbing material.

and thus make amore precise determination of the alteration of the penetrating power Obviously many other modincations and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated by the appended claims.

. I claim:

1. The method of investigating the formation around a bore hole which comprises lowering through the hole a source of gamma rays and neutrons, detecting the gamma rays scattered in the formation and returned to the hole to determine the density and atomicnumberof the formations surrounding the hole and simultaneously and separately detecting the neutrons scattered in the fonnation `dnd returned to the holeto determine the presence in the formation of substances containingjhydrogen.

2. The method of logging the formation sur rounding a bore hole which comprises passing through said hole a source of penetrative gamma rays whereby these gamma rays will en ter and be scattered in the formation.. accompanying ness of the gamma radiation may be ai'fected in the scattering process. Since the scattering is to some extent dependent upon the nature of the'formation penetrated, this change in penetrating rpower may be used to obtain additional information as to the nature ofv the formation. Figure 2 shows an arrangement for detecting this change in penetrating power. In this modi 3 fication 3l is a source of gamma rays or neutrons or preferably of both, and disposed above the said source with a pair of devices for detecting gamma rays emitted from said source and returned to the hole after being scattered in the formation, one of said detecting devices being surrounded with a layer of a substance having the property of selectively absorbing gamma radiation, and observing the amounts of scattered radiation picked up by said detecting devices as the source and detecting devices are passed 'continuously through the hole.

3. The 'method of logging the formation surrounding a bore hole which comprises passing through said hole continuously a source of penetrative gamma rays whereby the rays will penetrate and be scattered in the formation, detecting gamma rays scattered in the formation and returned to the hole. simultaneously separately detecting those rays scattered in the formation and which are less aifected by the scattering process than the mst-mentioned rays comparing the intensities of the rays detected and determining from the relative intensities the type of formation causing the scattering of the rays.

4. The method of logging the formation surrounding a bore hole which comprises passing #through said hole continuously a source of penetrative gamma rays whereby the rays will penetrate and be scattered in the formation, simultaneously detecting gamma rays scattered in the formation and returned to the hole by means of two adjacent detecting devices, shielding one of said detecting devices by a layer of a substance, such as lead, capable of selectively absorbing gamma radiation, comparing the intensities of gamma. rays and neutrons in said casing, a device in said casing for detecting gamma rays emitted from said source and scattered within the formation, a device in said casing for detect;- ing neutrons emitted from said source and scattered within the formation and shield members disposed between said source and each of said detecting devices. Y l

6. A device for logging the formation surrounding a bore hole which comprises a casing, means for lowering and raising said casing through said bore hole, a source of penetrative gamma rays and neutrons in said casing; a device .in said casing for detecting gamma rays emitted from said source and scattered within the formation, asecond device in said casing for detecting scattered gamma rays, a layer of a substance capable of selectively absorbing gamma radiation disposed around said second device, and a third device in said casing for detecting neutrons scattered in the formation and returned to the hole. 

